The foreseeable scarcity of fossil energy sources and the necessity to reduce the emissions of air pollutants and CO2 have led to a rethinking in the production of heat and the use of alternative, renewable energy, such as pollutant-free technologies, to ensure greater acceptance. An especially attractive form of heat production is the use of downhole heat. Downhole heat or geothermal energy is the energy below the earth's surface that is stored in the form of heat.
Even approximately 10 to 20 m below the earth's surface, the soil has an approximately constant temperature over the entire year, which increases with increasing depth. The natural temperature gradient of the soil is approximately 0.03 K/m of depth. This temperature is determined by the heat flux from the interior of the earth. The recovery of heat can be carried out by ground heat exchangers, which are embedded in a vertical, tight backfilled hole of, for example, 50 to 350 m of depth. A ground heat exchanger takes on the task of transferring sensitive heat energy present in the soil to a heat transfer medium that circulates in the exchanger, which medium transports the heat energy from the soil to the surface. There, the heat energy can be transferred to a second heat transfer medium that circulates in a heat pump.
In many cases, known ground heat exchangers are designed as U-tube exchangers, in which the heat transfer medium flows in a tube branch from the surface to the base of the ground heat exchanger hole; i.e., from top to bottom. In the other tube branch, the circulating heated heat transfer medium flows from the borehole base to the surface; i.e., from bottom to top. When rising, the heat transfer medium always releases a portion of the accumulated heat energy to the heat transfer medium circulating downward in the adjacent tube branch and to the surrounding colder soil. As a result of this undesirable release of heat, the exergetic efficiency of U-tube exchangers is relatively modest.
To improve efficiency, coaxial ground heat exchangers have been used. In the case of coaxial ground heat exchangers in the outer annular gap, the heat transfer medium flows from the surface to the borehole base, whereby it takes up sensitive heat energy that is present in the soil and flows back again through a central core tube upward onto the surface. If the central core tube is thermally insulated in such coaxial ground heat exchangers, the exergetic efficiency can be significantly increased relative to the U-tube exchangers.
While deep-reaching coaxial ground heat exchangers with insulated core tubes relative to the other known types of ground heat exchangers have a major exergetic advantage relative to the quality of the applicable heat flux, they also have the drawback of a large pressure drop of the heat transfer medium. That is, a large portion of the available cross-section of the ground heat exchanger hole is lost owing to the cross-sectional surface area of the heat insulation of the central core tube, owing to the relatively large wall thickness of the jacket tube of the coaxial ground heat exchanger, and in particular owing to the filling layer that is injected after the installation of the exchanger, for example, a mixture that can include (e.g., consist of) bentonite and cement, between the jacket tube of the exchanger and the surrounding soil. The net cross-sectional surface area for the transport of the heat transfer medium is thus greatly reduced.